Abaixo está uma instrução que descreve uma tarefa de geração de SQL, combinada com uma entrada que fornece mais contexto sobre os esquemas de tabela disponíveis. Escreva o código SQL que responda adequadamente à solicitação
 ###Instrução: Quantas classes de fones de ouvido têm um preço sugerido nos EUA de $150?
 ###Tabelas: CREATE TABLE réseau_du_sport_étudiant_du_québec (réseau_du_sport_étudiant_du_québec_id AUTO_INCREMENT PRIMARY KEY, University VARCHAR, Varsity Name VARCHAR, City VARCHAR, Province VARCHAR, Founded INT, Soccer Stadium VARCHAR, Stadium Capacity INT) CREATE TABLE current_line_of_headphones (current_line_of_headphones_id AUTO_INCREMENT PRIMARY KEY, Headphone Model VARCHAR, Headphone Class VARCHAR, Driver-matched dB VARCHAR, Construction VARCHAR, Earpads VARCHAR, Termination VARCHAR, US MSRP VARCHAR) CREATE TABLE 1952 (1952_id AUTO_INCREMENT PRIMARY KEY, Title VARCHAR, Series VARCHAR, Director VARCHAR, Characters VARCHAR, Release date VARCHAR, reissue? VARCHAR) CREATE TABLE executive_commissioners (executive_commissioners_id AUTO_INCREMENT PRIMARY KEY, Year VARCHAR, Human Resources & Operations VARCHAR, Local Affairs VARCHAR, Academic & University Affairs VARCHAR, External Affairs VARCHAR) CREATE TABLE amateur_draft (amateur_draft_id AUTO_INCREMENT PRIMARY KEY, Round INT, Pick INT, Player VARCHAR, Nationality VARCHAR, College/Junior/Club Team VARCHAR)  
SELECT COUNT Headphone Class FROM current_line_of_headphones WHERE US MSRP = $150